OP25j

There are a number of alternatives for hardware to capture a P25 baseband signal. These include:

• USRP (using TVRX daughterboard and appropriate antenna).
• SSRP with an appropriate radio front-end and antenna.
• Custom interface to existing scanner using soundcard for quadrature sampling.
• Custom PSK to serial interface hardware.

For those without any hardware it is possible to investigate using signal samples which are available on the DemodulatorPage.

=== USRP ===
Initial work is proceeding using the USRP which is available from [[http://ettus.com Ettus Research LLC]]. This is a relatively low-cost software radio (< 1000USD) that works with the GNURadio framework. An example is shown below:

Image(usrp.jpg)

The USRP is capable of wideband reception and requires no special-purpose hardware as everything is done in software. A USRP with the TVRX daughterboard (not illustrated) and an appropriate antenna is all that is required.

=== SSRP ===
The SSRP is a promising alternative to the USRP from [[http://oscar.dcarr.org/ssrp/index.php David Carr]] and is significantly cheaper. The sampling rate is reduced in comparison to the USRP and it is not nearly so mature but may be a good bet in future for mobile use or where the expense of the USRP cannot be justified.

=== Sound Card Sampling ===
The ADC built into many home computers is capable of sampling at rates up to 200KHz. Depending on the scanner it maybe possible to quadrature sample the discriminator output using the soundcard. This approach may require some custom interface circuitry and filtering within the scanner may limit reception to a single channel.

=== Custom hardware ===
Another approach for interfacing to existing scanners is to feed the discriminator output to a custom hardware circuit. Some are available built and tested from a variety of sources such as [[http://www.discriminator.nl/index-en.html discriminator.nl]]. This approach is low cost and pretty straightforward although limited to reception of a single-channel at a time.